This invention relates to the polymerization of macrocyclic polyester oligomer compositions. More particularly, it relates to the use of certain titanate esters as initiators for such polymerization.
The preparation of macrocyclic poly(alkylene dicarboxylate) oligomers and their polymerization to linear polyesters is described in U.S. Pat. Nos. 5,039,783, 5,214,158, 5,231,161, 5,321,117 and 5,466,744. The initiators employed for such polymerization include various organotin compounds and titanate esters.
Polymerization using these initiators is quite successful and affords polyesters having excellent properties and a wide variety of potential applications. However, to broaden the application areas in which the macrocyclic polyesters can be employed it is necessary to develop initiator systems of increased versatility.
For example, in structural reaction injection molding (hereinafter sometimes "RIM") processes a mold is initially charged with a fibrous reinforcement material and is subsequently filled by the introduction of two reactive streams. It is necessary for reaction to occur almost immediately and to be complete in less than 5 minutes, whereupon the molded part is removed and the charging operation repeated. For this purpose, therefore, extremely fast initiators are desired.
Conversely, in resin transfer molding (hereinafter "RTM") the two reactive streams are mixed outside the mold and the combination thereof is injected into the mold containing the fibrous reinforcement. It is necessary under those conditions to maintain the mixture in unpolymerized state until injection into the mold is complete, whereupon rapid reaction should occur. In other words, an induction period of measurable length, typically from about 15 seconds to about 2 minutes, is required. Merely decreasing the amount of initiator used is ineffective until very low levels are reached, whereupon polymerization is incomplete.
Another factor of concern is the level of initiator present in the polymerization mixture. Increasing the initiator level tends to increase the reaction rate and afford a more completely polymerized product. However, it also decreases the molecular weight of the polymer product when a tetraalkyl titanate, for example, is employed as an initiator, since each molecule of initiator initiates formation of an individual polymer molecule and generates four moles of alcohol which can function as a chain stopper.
Still further, the purity of the macrocyclic polyester oligomers can affect polymerization conditions and product quality. For the production of polyesters with the highest possible conversions and molecular weights, it is often necessary to remove linear carboxylic acid-terminated oligomers from the macrocyclics composition, as by passage through basic alumina in a column or the like. It would be desirable to eliminate this purification step without adversely affecting reaction rate or product molecular weight to any great extent.